Fix typo in comment

SLRU_SEGMENTS_PER_PAGE -> SLRU_PAGES_PER_SEGMENT

I introduced this ancient typo in subtrans.c and later propagated it to
multixact.c.  I fixed the latter in f741300c, but only back to 9.3;
backpatch to all supported branches for consistency.

src/backend/access/transam/subtrans.c

@@ -42,7 +42,7 @@
  * Note: because TransactionIds are 32 bits and wrap around at 0xFFFFFFFF,
  * SubTrans page numbering also wraps around at
  * 0xFFFFFFFF/SUBTRANS_XACTS_PER_PAGE, and segment numbering at
- * 0xFFFFFFFF/SUBTRANS_XACTS_PER_PAGE/SLRU_SEGMENTS_PER_PAGE.  We need take no
+ * 0xFFFFFFFF/SUBTRANS_XACTS_PER_PAGE/SLRU_PAGES_PER_SEGMENT.  We need take no
  * explicit notice of that fact in this module, except when comparing segment
  * and page numbers in TruncateSUBTRANS (see SubTransPagePrecedes).
  */